An Open Cycle gas core leaks highly radioactive fission waste, which gets worse the more you accelerate. Kerbal Space Command has decided you not allowed to use the Gas Core in the atmosphere or having the exhaust directed at the atmosphere of Kerbin. Therefore the trick is to use the Opwn Cycle Gas Core is to aim the exhaust always away or paralel to the Kerbin atmosphere.

The MIF is the first available fusion engine which is limited both propellant and isp range and requires an external power source to operate. On the plus site is cheap, light, available early and reasonable compact size (for a fusion engine).

Based on this document, I recently got a an idea for a new type of fission fusion hybrid which uses Fission Sail to boost the thrust and isp of a Deuterium Fusion engine.
The Engine would basically convert a large part of its fusion neutrons into useful thrust by using a sail (or parabolic mirror) coated with a thin layer of uranium 235/plutonium. It would be a good predecessor to the D-He3 Discovery Fusion Engine which is harder to achieve

Alright, this is the kind of information I need. What would be nice if there was a way to reproduce it. I have some vague ideas about how it was caused and I have already added some potential fixes but I'm not sure that fixes the problem. I guess we have to see.

Programming is indeed not nessisarry, you can simply replace the ModuleEngines by an ModuleEnginesFX configuration. it's just a matter of finding an exhaust effect and configuring and it should work. The trick is finding the exhaust effect.

Thermal power generation realy is fusions weak point, they are better at fuel economy and high isp. For the best thermal power generators you either use nuclear, antimatter fusion, positron antimatter or of course nuclear salt water!

Yes, there are several power generators: the thermal power generator , the MHD generator and the charged particle converter. Depending on the reactor you can or have to use different power generators. The compatible types can be found in the part info.

Notice there is now is a nuclear alternative to Antimatter engines which is the Nuclear Salt Water Rocket Engine. Its even more powerful, but can only be used at full power and you need to be in high space or off-world to be allowed to use it due to the highly radioactive exhaust. It's ideal for heavy interplanetary multi-mission motherships

replace the beam core antimatter reactor by positron antimater reactor, replace charged particle generator by MHD generator and the replace antiproton container by an positron container and you should be good to go

Well the Fission Fragment reactor fulfills indeed a niche in the sense it is the first Fission Reactor capable of producing high energy charged particles which can be utilised for either plasma or direct magnetic nozzle propulsion, producing the highest isp of any fission reactor or for direct power generation, allowing the highest possible electric efficiencies possible with fission technology. Of course, fusion technologies is potentially better in both roles. So the fission fragment reactor is kind of intermediate technology that gets surpassed by fusion and therefore eventually becomes a dead end technology. Good Scenarios when to use a fission fragment reactor is when you need higher isp then Open cycle gas core reactor can provide (7000s) or when you want to minimize waste heat for electric power production and made no or minimal advancement in fusion reactor technology. Power is nice but isn't everything. I would say that its propulsion and electric efficiencies make this reactor pretty useful, don't you agree?

The Magnetized Target Fusion Reactor is kind of the jack of all trades, master of none kind of reactor, it can do a little bit of everything, but nothing spectacular. This second generation fusion reactor is the first fusion reactor that can be scaled down to 1.25m and is the cheapest fusion reactor available. Its main advantage compared to the pebble-bed reactor is endurance and fuel cost, as it uses very little fuel at 100% fuel efficiency and can run on cheap fuel like deuterium. All of this might not be obvious so I will add it to the description.